1. Field of the Invention
The present invention relates to a process for preparing polyimide and more particularly, relates to an improved process for preparing polyimide having good thermal stability.
2. Description of the Related Art
Polyimide obtained by the reaction of diamine compounds with tetracarboxylic acid dianhydride is excellent in mechanical strength and dimensional stability in addition to exhibiting high thermal resistance, flame retardance, electrical insulation, chemical resistance and other characteristics. Hence, polyimide has conventionally been used in many fields such as electric and electronic members, space and aeronautics parts and transportation equipment. Many preparation processes of polyimide have been investigated.
For example, Japanese Patent Publication SHO 36-10999(1961) proposes a process for preparing polyimide by reacting a diamine compound with a tetracarboxylic dianhydride in an amide-based solvent to obtain a polyamic acid, the precursor of polyimide, and then carrying out chemical imidization by using imidizing agents such as acetic anhydride to prepare the polyimide having a recurring unit corresponding to the polyamic acid.
The process is unfavorable from the industrial viewpoint that a long time is required for the polymerization of polyamic acid which is the precursor of polyimide and storage stability of the polyamic acid is poor. Acetic acid is formed in the imidization reaction and is difficult to remove from the desired product polyimide. When polyimide is molded in the presence of acetic acid, problems such as foaming take place.
As to a method for solving these problems, a method of preparing polyimide by reacting a diamine compound with a tetracarboxylic acid dianhydride in a phenol-based soIvent such as cresol and conducting heat dehydrating reaction to obtain polyimide has been proposed in F. W. Harris et al, Applied Poly. Symposium, No. 26, p421-428(1975) and Japanese Patent Publication SHO 58-4056(1983).
Such process can improve problems that occur in the process where polyamic acid is formed in the phenol based solvent and successively imidized or where polyamic acid is formed in the above amide based solvent and successively subjected to chemical imidization in acetic anhydride.
The process proposed by Harris et al is to prepare a polyamic acid-cresol solution by using m-cresol as a solvent and isoquinoline as a catalyst. However, tetracarboxylic dianhydride used in the process is phenylated bisphthalic anhydride having a specific structure represented by the formula (1) or the formula (2) : ##STR1##
No example has been known disclosing the use of tetracarboxylic dianhydrides having low solubility in solvents, for example, pyromellitic dianhydride and benzophenonetetracarboxylic dianhydride which are usually used in industry.
These process also have the disadvantages that oligomers are formed by the difference of solubility in the phenol-based solvent between the diamine compound and tetracarboxylic dianhydride and contamination of oligomer impairs the thermal stability of polyimide, and that structurally modified specific tetracarboxylic dianhydride are required in order to enhance solubility in the phenol based solvent.